This enables users to add a signature to all blog posts, and update it from a single location.

Requires database migration with: writefreely db migrate

Closes T582

Ref T627

from /mention:

Ref T627

by making them absolute, not relative.

Ref T519

- return an error with invalid request types
- simplify json decoding
- return error and success consistent with app conventions
- endpoint change from /api/generate/markdownify to /api/markdown
- fix nil pointer dereference when passing a base_url

Previously, links would go to /user/mention:@me@this.tld instead of
/mention:@me@this.tld

intermediate page (WIP) that shows the user profile page url

this allows abbreviation elements to keep their title attributes when
containing special characters.

GetCollections and GetPublishableCollections now take a hostname
parameter to allow setting the collecion hostname.

All collections used in memory now have their hostname set.

instead of linking to posts only on a user's blog.

Ref T681

This whitelists the HTML5 <audio> element and adds some basic style to
make it look nice.

Previously, our hashtag parser would indiscriminately replace
hashtag-like text with hashtag HTML -- including in places it shouldn't
have, like inside code blocks. Along with the v1.7.0 changes to
writeas/saturday, this fixes that and closes #6.

As a bonus, strings of #spaceless#hashtags#in#a#row are now rendered
correctly.

This improves rendering in a number of situations:

- it keeps anchor tags working
- it gives the user some control for not linking, for example in code
  blocks.

Con:
hashTags at the beginning of a line without a space won't get linked.

Workaround related to issues #42 and #6 and #33

- New editor nav
- New backend nav
- Support for drafts
- Different footers on backend

This includes config changes, collections, posts, some post rendering
funcs, and actual database connection when the server starts up.