Rename "user" to "member" throughout codebase

Phew, this one's epic.  A major refactoring to standardise on "member" throughout the codebase, plus some related namechanges in other tables.

First off, the basic s/user/member/g situation:

* User is now Member
* most things looking for user_id now look for member_id
* methods generated by devise now refer to member, not user. Eg. current_member and member_signed_in? rather than current_user and user_signed_in?

After that change had been made, we now had fields like Garden.member_id and Post.member_id.  These names were clunky, so I renamed them.

* Post.member_id became Post.author_id, so you now refer to post.author
* Garden.member_id became Garden.owner_id, so you now refer to garden.owner -- in fact we had a convenience method for this before, but now it's official.

I also did general cleanup when I came across something that needed it.  Mostly this is just in the tests.

I've tested all this pretty thoroughly -- rake passes (though at one point, I had some trouble with my test db and had to blow it away and start fresh; that seemed to be just some weird effect of me doing too many migrations or rollbacks so hopefully it won't affect you.)  I also did some pretty extensive testing via the website for each stage of the changes.  I've confirmed that signups and logins still work, as does posting and creating gardens and planting things and all that.  The RSS feeds for members' updates also still work (tested with curl).