[webui] remove the rest of the engine

also make sure the API routes to not try to reply to HTML - which only
gives crashes
[webui] global catch for invalid :back

if people visit invalid URLs without coming from links of ours, then
they just get the main page.
[webui] remove the README.theme - the information is outdated

with pipeline and Henne's recent changes anyway
[webui] adapt the asset path to henne's move
[webui] invalid project _meta cache if editing repositories

Fixes #485
[ci] remove obsolete jenkins files

do not cat the log file, it makes the problems hard to find
[webui] remove unused helper
[api] little more coverage