When there are no filters, we can get the total todos count from the cached
count on the user object, instead of performing a DB query. This query takes
about 80ms for me on GitLab.com.

This solves an N+1 query problem where we'd run multiple queries when
getting the namespace owners of the displayed projects.

This changes various controllers to use the new EventCollection class
for retrieving events. This class uses a JOIN LATERAL query on
PostgreSQL to retrieve queries in a more efficient way, while falling
back to a simpler / less efficient query for MySQL.

The EventCollection class also includes a limit on the number of events
to display to prevent malicious users from cycling through all events,
as doing so could put a lot of pressure on the database.

JOIN LATERAL is only supported on PostgreSQL starting with version 9.3.0
and as such this optimisation is only used when using PostgreSQL 9.3 or
newer.

This solves an N+1 query problem where for every project we'd query the
creator separately just to figure out what avatar to display.

TodoService should not call `.select(&:id)` on todos, because this is
bad performance. So instead use sub-queries, which will result in a
single SQL query to the database.

https://docs.gitlab.com/ee/development/sql.html#plucking-ids

[9.3 security fix] Renders 404 if given project is not readable by the user on Todos dashboard

See merge request !2118

Refactor atom builder by using a layout

See merge request !12114