admin: Fix selection of default catalog view

Return the highest-order view type in the `ADMIN_VIEWS_ORDER` setting.
admin: Add URL test for Admin

Add a test that checks if the `ADMIN_BASE_URL` setting is used by Admin
and if we have any issues with slashes and redirections.
admin: Add tests for default catalog view
docs: Update install/admin guides

Update the install/admin guides to reflect the addition of the
`ADMIN_BASE_URL` setting. Also, add an Admin section in the CentOS
install guide.
deploy: Add `ADMIN_BASE_URL` in Admin conf

Add an `ADMIN_BASE_URL` setting in the admin.conf file. The value of
this setting is created in the same fashion as the values for
CYCLADES_BASE_URL / ASTAKOS_BASE_URL.
Mention the `ADMIN_BASE_URL` setting in Changelog