activities_js: remove dead code (out-of-date prototype)

This dead code tripped my grep for 'admin'.  I don't try to update it,
I just remove it so it doesn't turn up in grep.

As well as hardcoding  'admin' (should be SITE_AREA),
this wouldn't work without adding ci_csrf_token
(see Bonfire 0.6 changelog).
Fix remaining hard-coded /admin links (affects database module)

Replace hard-coded 'admin' with SITE_AREA.

These links also omitted to use site_url (for sub-directory installs).
Using site_url directly would leave them a bit messy, so I used anchor()
instead.
database module: fix a couple of broken links

Links were missing the context segment (developer).
AUTO_INCREMENT portability fix (#750)

AUTO_INCREMENT isn't portable SQL.  dbforge converts it to different types
in different databases, and they don't all work the same.

Inserting 0 or NULL into an AUTO_INCREMENT column is MySQL-specific.
It doesn't work in PostgreSQL.  Use the obvious portable approach of not
assigning a value to the AUTO_INCREMENT column.

It seems this approach obvious to most other people as well :).
This only affected one migration mentioned in issue #750,
and the creation of new roles.

There was similar-looking code in users and builder which shouldn't affect
anything, but I've tidied it away so we can be sure.
Merge branch 'develop' of github.com:ci-bonfire/Bonfire into develop