fix #764 : implement support for external links in sidebar

[rasapetter]

Closes #764

Summary
We introduce a new type of sidebar item called 'external' which are
treated differently in Sidebar/SidebarLink. These items are rendered
as elements with an added icon indicating that the element points
to an external location.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox v63.0
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[rasapetter]

Any chance of this making it into alpha.26 or should I approach this from another angle?

[alessandromenini]

+1 for this!
Is there any release date?

[timaschew]

Is there any other work to do before this can be merged? @ulivz

@rasapetter can you please rebase?

[timaschew]

@ulivz Any plans to merge this soon?

[narration-sd]

+1 if you can, on the merge - I'm another who needs this.

The case is an obvious one:

• I have a project coming out, worth the long effort
• The doc needs to be referenced from a main/etc site - on Gridsome - Vue
• There needs to be a way back to the main site. You'll understand.
• Thank you!
• Closer integration yet would be the future, but this is great for now

p.s. what I'm doing should soon be interesting to Gridsome and Vue

[timaschew]

The problem is the branch contains conflicts and it's even not working. I've tested it with the latest version. I've found the missing piece to get it working "again".

I've actually an additional use case which I would like to pick up in case I will send my PR.

This is my use case:
I want to provide some HTML, actually API reference which should be part of the VuePress build. Actually the files are just in the public directory, not handled by VuePress at all. But since they using the same basepath links to those files are "relative" (starting with a slash). So VuePress is printing an error because those files are invisible to VuePress. But there are working fine and handled as external links.

What do you think?

[narration-sd]

@timaschew very good to hear your progress, is the first thought, and a wish I wasn't so occupied that I could jump in on this.

It's a basic need, wherever the routers of these SPA systems get hold of browser targetting -- and should be very solvable via the router access itself. It can be stopped in its tracks via the callbacks, and that's where I'd think to intervene, and presume this does, with too many other codebases open to give this one the attention. The doctor is operating...patient gets first care. We hope...!!

I don't quite understand your use case, but appreciate, esp. the difficulty across languages; long life in Europe. But I'm sure it is quite sensiblle, and that many others are quietly mystified that there is this problem. We all tend to design first for the problem we think we are solving, and that's what happens with these powerful and focused applications..

Best, then, and encouragement -- and thanks.

[timaschew]

Let's do step by step: #1534

[ulivz]

Closing in favor of #1534