Add frontend support for inline spoiler text

Add frontend support for inline spoiler text

To get around the fact that React manages the contents of statuses
through HTML injection, this commit adds a Redux action/reducer for
dynamically mutating the content of a status in the store. This
approach has some flaws:

- It’s messy as fuck, and

- If the status is reloaded for whatever reason (i·e it is received
  again through the A·P·I), the contents won’t match and there will be
  *another* (unnecessary) status body update (which will close any
  open spoilers in the status).

Still, I think this is the best that can be done with Mastodon’s
current architecture.