Fixes #429.

Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

The changes are a bit heavy since the emojis were loaded into module
attributes from filesystem.

This introduces a GenServer using an ETS table to cache in memory the
emojis, and allows a runtime-reload with `Pleroma.Emoji.reload()`.

This reverts commit d31bbb1c, reversing
changes made to 340ab3cb.

* Pheonix has an extra scheme whitelist conflicting with ours
* Pheonix doesn’t seems to do URL encoding, just HTML encoding

Closes: #307

default that returns empty

This reverts merge request !309

formatter: adjust regexp captures to match local users with single character nicknames (closes #163)

when generating hashtag links, we used the casefolded version that we use in
the link URLs, instead of the original version.

accordingly, adjust the formatter to use the original text for the links, while
keeping the casefolded version for the URLs.