Pull request quality assurance

When checking if PR could be merged or not the following criteria should be considered among others:

• There should be either an issue linked to PR or PR should have a good description on what is it fixing or adding.
• Unit tests. Not mandatory but very appreciated. These should fail without the code that the PR is fixing.
• CHANGELOG entry should present. It should be put into next release section, ordered by issue type and number. Nicknames of people responsible should present.
• Code style and views code style should be OK. These could be fixed while merging if the person merging prefers it that way.