When DVCS was implemented, the diffviewer model gained a lot of new

models, and with those came ModelAdmin and InlineModelAdmin

definitions. Many of these were making use of the default dropdown

fields for relations, which wouldn't scale in production. To solve that,

we needed to add more fields to raw_id_fields. On top of that, we

never actually had this set for the inlines, meaning we needed to do

that there as well.
This change takes the approach of defining all the attributes common to

both the ModelAdmin and InlineModelAdmin for each model type into

private _*Common classes, and then mixing them in to both classes for

the model. This ensures we don't regress in the inlines, and speeds up

loading all around.
It also implements some future-proofing for the diff and parent_diff

fields for display of a FileDiff, removing allow_tags and replacing

with a return of SafeText for each. This will be needed eventually for

Django 2.0+.
I also found that we were setting the label attribute on both of

these, which doesn't exist. short_description was the correct

attribute to set, which we're now setting.

Tested on Django 1.6 and 1.11 with viewing a FileDiff, DiffSet,

and DiffCommit.