WIP: Feature: add cw/ca/spoiler/summary (help requested)

This branch currently adds support for a content advisory (or something). It can submit statuses with content warnings and addresses #69 (closed) . Mastofe current respects them and pleroma-fe displays them like they normally are.

There are a few things that need to happen though:

• Come to a consensus about what this should be called in the interface/what the button should be
• Localize the two new messages
• Optionally keep the spoiler in replies
• Add toggles to turn this feature on or off (maybe this is a separate merge request?)
• bonus, switch the error messages in post_status_form.js to use the localization file
• A related item: hide text if there's a spoiler with a toggle. Should probably also be optional
• Turn all this functionality off if the back-end is GNUSocial

One kind of weird thing I noticed is that pleroma stores summaries, but mastofe/common API expects spoiler_text. If need be, I can change the frontend code to talk about summaries for consistency, but I think this will then require a change to pleroma to handle that translation in Common API.

Is it abusing the twitter API that I added this extra form field? I wasn't sure how else to do it.