Support for branches & tags in SVN repositories

Supported:
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- SVN branch/tag discovery ("Subversion > Layout" setting) must be enabled manually for each repository
- SVN branches in "diffusion.branchquery" Conduit call
- SVN tags in "diffusion.tagsquery" Conduit call
- SVN branches/tags are recognized as Refs in a repository with all associated functionality (e.g. showing branches/tags on repository home page, ability to browse a specific branch/tag only, etc.)
- names of actual folders for "trunk", "branches" and "tags" can be set what's actually used in the repository in "Subversion > Edit" section
- display of branches/tags on commit detail page

Not Supported:
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- import only commits from tracked branches (right now all commits are imported)