nix: Add methods to support develop installations from config.

nix: Move shell hook: default.nix -> shell.nix and set custom prompt. When using multiple dev envs with nix-shell it's good to have a unique shell propt to distinguish them.

validators: fix glob validator raise statement misplacement.

files: fix loading of source files events via pjax.

backends: don't detect backends when initializing db based vcs-instance. This skips some calls back to vcsserver.

vcs-http: explicitly pass in vcs-type into vcsserver.

simplevcs: fix a typo when running detection of vcs backend. - the flag should be explicit_scm which skips detection of VCS systems. This improves the speed of calls by not trying to look for a valid vcs.

system-info: added info about temporary storage. - temporary storage is used by Mercurial and other parts of the system. knowing it's limitations is important to know.

tests: use WARN loglevel by default, and add --test-loglevel flag to change this. Reduces the ammount of logs printed during tests.

tests: sanitize temp group name to conform with our schema validators that clear the special chars out.