Partitioning: Initiate the web_hook_logs partitioning

The first step for partitioning the web_hook_logs table (&5558 (closed)) is to create the initial partitions (covering existing data), add the trigger that will keep the partitioned table in sync with the original table and schedule the background migration jobs that will copy all the existing data.

In detail, the following will be covered:

1. Create a new partitioned table with a nearly identical schema as web_hook_logs. The only differences will be that the primary key is bigint (to address #276021 (closed)) and that the indexes will be missing (to facilitate faster inserts).
2. Create all the monthly partitions that cover the existing data.
3. Install a trigger that keeps tables in sync - in particular it mirrors inserts, updates and deletes into web_hook_logs to the newly created table
4. Schedule background migration jobs to copy data from web_hook_logs to the partitioned table
5. Add explicit primary key definition to WebHookLog. This will not affect the current behavior of the application, but will facilitate the smooth transition when we are going to swap the tables two milestones after the partitioning is initiated.