application website validation, don't link to app website if website isn't set,
also comment out animated boost icon from #464 until it's consistent with non-animated version

with YAML-defined defaults. Add admin page for editing global settings. Add "site_description"
setting that would show as a paragraph on the frontpage

This allows other platforms such as Windows, macOS and iOS to use their system fonts rather than downloading a copy of Roboto. It also makes the app feel a little closer to native on those platforms!

The "settings" links in the Getting Started section (or, if #399 were to happen, "more" menu) and compose sections are now different; the "compose" link is "Edit profile," while the one in the Getting Started section is now "Preferences."

All languages have been updated to accommodate this, based on the existing usages of these phrases in language files in the Rails part of the app!

addresses part of #384

Don't ask users to check the console - if they're on mobile, they probably can't anyway ;)

metaKey is only set correctly on keyDown, not keyUp, so this swaps to using that

This stops the empty compose view from scrolling on 4-inch devices.

A couple of places were using "reblog" rather than "boost" - this updates them to match the web UI

Removes extra UI margins < 360px, and allows the tab bar to scroll. Also slightly improves horizontal scrolling behaviour on desktop.