Implement automatic inclusion of hashtags

Implement automatic inclusion of hashtags

With the increased importance of hashtags in the Mastodon world, make
the support more efficient and useful. This adds the ability to flag a
hashtag as "auto add", in which case it will be automatically added at
the end of all posts giong out to twitter/mastodon as a result of news
posts (this was not possible at all before). Additionally, it will also
get pre-filled in on manual posts made both using the backend interface
and the mobile webapp.

To control the order of appearance of the hashtags, a sortkey field has
been added to the table as well. Existing hashtags will just be sorted
alphabetically.