chore: index dates and mr events

[dejan-crocoder]

Resulting query plan for select events in week by merge request:

ID     PARENT     NOTUSED     DETAIL                                                                   
6      0          0           SEARCH d USING COVERING INDEX dates_week_idx (week=?)                        
10     0          0           SEARCH ev USING INDEX merge_request_events_occured_on_idx (occured_on=?)     
15     0          0           SEARCH u USING INTEGER PRIMARY KEY (rowid=?)                                 
18     0          0           SEARCH mr USING INTEGER PRIMARY KEY (rowid=?) 

[ivke995]

LGTM