Provide basic support for Subversion revprops

Summary:
Ref T11208. See that task for a more detailed description of revprops.

This allows revprop changes in a hosted Subversion repository if the repository has the "allow dangerous changes" flag set.

In the future, we could expand this into real Herald support, but the only use case we have for now is letting svnsync work.

Test Plan:
Edited revprops with svn propset --revprop -r 2 propkey propvalue repositoryuri:

• Tried before patch, got a "configure a commit hook" error.
• Tried after patch, got a "dangerous change" error.
• Allowed dangerous changes.
• Did a revprop edit.
• Prevented dangerous changes.
• Got an error again.
• Made a normal commit to an SVN repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11208

Differential Revision: https://secure.phabricator.com/D16174