Add Heroku deployment support

Show 'do not display in public timeline' when replying to self

Change boost sprite to look like fa-retweet

Fix S3 default settings

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

Make boost button spin around

Add a 'Back to Mastodon' link to footer of settings pages

Hide 'Do not display on public timeline' for reply toots

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

fix Vagrantfile so that the correct ruby is installed

Prior to this commit, the vagrantfile installs ruby 2.3.3. Due to commit mastodon@7951e7f, this no longer works.

Change the provision scrip so that 2.3.1 is installed via rbenv. Note this drastically increases provision time.