[backend] link_or_copy: unlink the target before linking

Otherwise the code will truncate the file in the cache
if the link already exists for whatever reason.
[backend] do not postprocess for projects of another sibling
[backend] change 'unresolvables' to 'unresolvable' in scheduler log
[webui] remove the workaround for old webkit issue

https://github.com/thoughtbot/capybara-webkit/issues/444
[backend] don't do setchanged for package source changes on another partition
[webui] random fixes from crash reports
[webui] test replying to comments
[backend] add random component to 'rescan' timeout
[backend] do not upgrade remote project events
[backend] add a way to signal that a project on a different partition is gone
[backend] add "withchangesmd5" parameter to package info query

Done so that the API needs less requests to gather the information
it needs.
[backend] enforce sha256 for newer rpm-md repositories
[api] refactor status controller
[api] refactor one more function
[api] srcmd5 should probably not be a class variable but an instance variable
[api] replace linked_packages with backend_packages

Also saves md5sum for all packages not just links
[activexml] do not retry failed connections

This was needed back in lighttd days
[backend] do not use xrpc_setchanged for linked packages

setchanged uses an unfair unshift to bring the prps to the front.
[api] send delete comment notifications to hermes