maintner, devapp: initial integration of maintner

+ Add UpdateWithLocker method to maintner.Corpus to allow an
  external locker to be passed that will be held during mutations.
+ Add OwnerID to maintner.GerritCL that returns the Gerrit user ID
  of a change using its meta commits.
+ Add maintner corpus to server.
+ Add /imfeelinglucky and /imfeelinghelpful endpoints that redirect
  to a random HelpWanted GitHub issue.
+ Add initial scaffold for GopherCon dashboard.

Change-Id: I2567b084f012cad5a9b4dee07e92be05088ac9d8
Reviewed-on: https://go-review.googlesource.com/47830
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
8 files changed