Add a 'default' network option that changes modal into split dropdown

[dsevillamartin]

Fixes #29

Changes proposed in this pull request:

• Add option for default share option
• Switches layout to SplitDropdown if share option is supported (i.e. if native is selected & supported)
• I tried to use a custom component for ShareButton but SplitDropdown replaces the component class for the active one... so had to resort to using <Button> directly

Reviewers should focus on:

• If there's only one share option and it is made default, it'll just say the network's name, eg. Twitter, which I don't love. I'm not sure how to improve that though - adding more text like "Share with" is too long except in phone layout
• I made the dropdown buttons look separate like the modal to keep the style similar and differentiate from just selecting an option like the following dropdown

Screenshot

• Native & Twitter enabled, with native as default (Edge vs Firefox):
  
• With Twitter as default (Edge vs Firefox vs Phone):
  
  
• Native default w/ more alternative options
  

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).