Update README.me setup instructions

Fact-checked every command
[api] start to move maintenance logic into API models
[webui] remove the issues from the incident table

They need to be revived - I put them in the project/_incident_issues
partial, but it's not yet used
[webui] restore request flags in maintenance_incidents
[webui] remove some images that appear unused
[api] remove unused variable
[backend] use defined host variable

This fixes the many warning messages during test suite run
[api] reducing SQL changes and avoiding dead locks

Only really needed changes are applied to the database when running issue tracking
on changed sources.
[api] beautify the code a bit
[api] turn all errors in do_branch into exceptions.

- adding generic project name validation method
- This is changing in two relative obscure cases the return error code, but to the correct value.
- Moving more generic errors to the exception lib and rename some.
Get rid of the obs_import section, doesn't work anymore
[backend] bring back the BUILD_UNCHANGED event for api build status tracking, but do not deliver it to hermes.
[webui] reduce usage of WebuiRequest model