[Buildbot] #3188: add a 'slug' to change sources so they can be referred to by name

[Buildbot trac]

#3188: add a 'slug' to change sources so they can be referred to by name
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Reporter:  dustin       |      Owner:
    Type:  enhancement  |     Status:  new
Priority:  minor        |  Milestone:  0.9.+
 Version:  master       |   Keywords:
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 Blocks #2635.

 Add a new, automatically populated, unique identifier column - a slug - to
 the changesources table, and allow users to specify that slug in URLs in
 place of `n:changesourceid`.

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Ticket URL: <http://trac.buildbot.net/ticket/3188>
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