[api][webui] make maintained projects a model

This fixes some database issues. Brought to you from OhgottOhgottOhgott-Releases....
[backend] fix buildenv generation if the worker uses a preinstallimage
[api] do not allow random users to declare to maintain projects anymore

This is not a security problem, but it can create confusions when too many
projects claiming to maintain certain projects
[backend] worker: do not put src rpms into the generated _channel

Also put a warning about conflicts in the _channel file if a
binary comes from multiple sources.
[backend] readd slepool hack, at some time we will have a decision

This reverts commit 55aaf022b281a3f9943cf6600223e0b58b636ec3.
[api] support release of packages with specified target
[backend] fix meta file sorting for kiwi images

Commit #92ae8762 broke it...
[backend] one sort is enough
[backend] publisher: fix stage syncing for redirected projects
[backend] adapt publisher to better support top-level trees
[backend] fix triggerdir substitute

Perls ability to accept broken syntax never ceases to amaze me...
[backend] clean up extdir handling in sync_to_stage
[backend] bs_srcserver also set debug flag for debuginfo pool channels
[backend] bs_srcserver for pool it is called medium not name
[backend] collect checksums when rebuilding a channel
[backend] publisher: implement updateinfo filtering for splitdebug
[backend] publisher: change tbz handling for kiwi images
[api] maintentence branch: do not copy from devel project if we're branching a specific revision
[backend] publisher: rename main to mainupdateinfo
[backend] support gem files in source diff