Restricted permission to start a new discussion is flawed

[pflstr]

Bug report

• Version of Flarum: 0.1.0-beta.7
• Website URL where the bug is visible: https://devflarum.xyzz.work

Additional comments

I set the permissions, so that globally all members are allowed to start new discussions, but only e.g. mods are allowed to do so in a specific tag. If a non priviledged user enters said tag, the button Start a discussion looks as if one where allowed to do so (I would expect Can't start discussion). If the user then tries to submit such a discussion, he can't do that being noticed You do not have permission to do that.

If the same settings apply to reply to discussions, everything is fine (Can't reply).

[askvortsov1]

Duplicate of #1696 and #2149