administration: add user management panel

[kpsherva]

The end goal of this task is to have the administration panel for the user management. This task is considered completed when below steps are achieved:

• check on how the invenio-administration module works
• add user list page to the administration panel - find and use the search endpoint (and as a consequence resource, since resource can be used in invenio-administration) which provides all the users so that administrator can see all the users. Things to consider: is the endpoint displaying all the information? Is it hiding any users? do we need special permissions to make all the users information available to the administrator. HINT: superuser != administrator. These are important points and should be discussed with the team before starting the implementation
• implement the list view
• implement the user details view
• add a feature flag APP_RDM_USER_ADMINISTRATION which will conditionally display or hide these views in the administration panel (similarly to COMMUNITIES_ADMINISTRATION_DISABLED)

IMPORTANT! The scope of this task includes only displaying the available information - do not add any new fields visible in the mockup to the user or any actions (buttons). These steps will be added later as next iteration.

List view:

Details view

[psaiz]

inveniosoftware/invenio-users-resources#82 addresses the first four checkboxes. Let me take a look at the last one before moving the ticket to the next column

[psaiz]

Ready for the second round of the review. Note that this is still a draft