This commit optimizes the page loading of this page in several ways:

1. Remove exact counting of projects on the page. By default, a `SELECT
COUNT(*) FROM projects` would be called, which could take seconds on an
instance with many projects.

2. Preload project/namespace/routes where necessary. This reduces a
significant number of N+1 queries.

3. Remove the use of `becomes(Namespace)` because this causes a SQL
query each time it is called. Instead, use the Rails path helpers to
generate the path directly via `project.namespace`.

Closes https://gitlab.com/gitlab-org/gitlab/issues/119093