Rename internal search filters (with deprecation warnings)

[Flix6x]

From https://github.com/FlexMeasures/flexmeasures/pull/451#discussion_r952887454:

The distinction between the timing of beliefs and the timing of events is critical to our data model, and we welcome tweaks to make it more understandable. I expect we'll have to deal with some deprecation warnings in various plugins, though. Filters I'm considering renaming:

• beliefs_before -> beliefs_formed_before
• beliefs_after -> beliefs_formed_after
• event_starts_after -> events_occurring_after or events_happening_after (in timely-beliefs, this should become event_ends_after)
• event_ends_before -> events_occurring_before or events_happening_before (in timely-beliefs, this should become event_starts_before)

[Flix6x]

Relevant timely-beliefs PR: https://github.com/SeitaBV/timely-beliefs/pull/120