On submitted revdel form, prefill selected reason dropdown

When you submit a revdel form, the 'other/additional' reason will be in the
returned form. However, the common reason dropdown always defaults to 'none'.
This commit makes it use the previous common reason option.

Bug: 48508
Change-Id: I5b16aef33bdbabbc9608faac7f090f8dadcf79d4
Merge "On submitted revdel form, prefill selected reason dropdown"