Add timelog categories table and model

What does this MR do and why?

Related to #20842.

This is the first MR introducing the work on timelogs categorization in order to support their billability, etc.

With this MR the new timelog category model and its relative table are introduced.

Screenshots or screen recordings

== 20220513095545 CreateTimelogCategories: migrating ==========================
-- create_table(:timelog_categories, {})
-- quote_column_name(:name)
   -> 0.0000s
-- quote_column_name(:description)
   -> 0.0000s
-- quote_column_name(:color)
   -> 0.0000s
   -> 0.0150s
== 20220513095545 CreateTimelogCategories: migrated (0.0151s) =================

== 20220513095545 CreateTimelogCategories: reverting ==========================
-- drop_table(:timelog_categories)
   -> 0.0028s
== 20220513095545 CreateTimelogCategories: reverted (0.0029s) =================

How to set up and validate locally

Applying the migration should be enough.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.