Improvements to UX for non-logged-in instances

[jcgurango]

Some UX improvements for what happens when you're browsing an instance that isn't your home instance. This will consist of multiple PRs and this issue is for tracking them and discussing possible additional improvements or alternative implementations.

Planned features:

• Menu item to view a community in a specific instance other than the originating instance #254
• Menu item to view comment or post in a specific instance #277
• Shortcut to post/comment directly to your home instance's version of a post/comment
• Subscribe from your home instance when viewing an instance
• Ability to vote using X account when browsing another instance's post

[jcgurango]

Before the commenting stuff happens I think we should look into a fix for #118 and #246 as this will tie into the "view comment on instance" idea.

EDIT: Also #117 will be important for that.