per_conf: provide additional config validation

[okt-alexanku]

Current config validation just checks that submitted JSON can be parsed.
No more consitency checks are performed on backend side.

Please provide some additional checks that configs can be applied.

1. Meta config validation
   meta_categorization task starts when meta config is saved. It can fail, but user can't know this.
   Recent bublik update from v1.9.0 has shown that meta_categorization task may fail on re.compile() for some of set_patterns

2. Per_conf validation
   Per_conf update may break dashboard.
   Recent bublik update has shown that backend returns 400 bad request
   when some of DASHBOARD_PAYLOAD fields are missing in the DASHBOARD_HEADER definition.

3. References config validation
   Perhaps, basic validation that uri is http/https link.
   I'm not sure if it's possible, but warning about existing refs in the DB that are not listed in the references config might also be helpful.