Add 'changes_file' key to release data

[rwstauner]

I made a comment to this effect when I added the changes controller:

e671469#diff-4ed60210a2ea835de90db16374d32449R18

If we did store the path to the change log on the release object
we could use that to fix metacpan/metacpan-web#1107.
We'd know ahead of time whether or not to make the link
and we wouldn't have to guess which file it was when trying to return it from the changes controller.

Note that this would require reindexing everything (#289) to be useful.

[oalders]

This is tagged as API V1, but since we're adding a field and not altering or removing any other fields, I think it's fine for v0.

[mickeyn]

most releases have this data, but it's not used in the changes controller - need to replace the hackish query to make use of it.

[ranguard]

@mickeyn not sure what's the score with this ticket?

[ranguard]

Just found it's needed for metacpan/metacpan-web#1107