Reorganize admin pages/tasks

[vitoravelino]

Description

Currently admin pages are kinda confusing and not too useful. For example, an admin cannot create a team when they reach the teams admin page. The same happens for namespaces. The repositories page currently list all the repositories you have access instead of only your own (like we do for namespaces and teams).

Based on those issues, we decided to unify the admin tasks into the normal pages. So we will get rid of some admin pages and improve the rest

• Get rid of admin teams page and unify with normal teams page (via teams: improved team creation form with owner #1776)
• Get rid of admin namespaces page and unify with normal namespaces page (via namespaces: unified admin page with regular page #1783)
• Split repositories into "teams member repositories" and "other repositories" (via repositories: splitted repositories into different panels #1785)
• Migrate admin dashboard overview elements (number of teams, namespaces, etc) to normal overview (via dashboard: unified admin dashboard page with regular page #1786)
• Group recent activities into "personal" and "administrative" tabs or something similar (via dashboard: unified admin dashboard page with regular page #1786)
• Show "users" sidebar item if admin (via sidebar: added users and registries #1784)
• Show "registries" sidebar item if admin (via sidebar: added users and registries #1784)