Move the usr_roles table to usr_access_roles as that is what it relates to

[Robert Lyon]

We use the table via a get_user_access_roles() and it involves the roles relating to user access rules so we should name the table to reflect this.

This also allows us to add a usr_roles table in the future to allow more granular rules for a user, eg can do some site admin options but not all

To test:
- One should be able to do peer assessment like before and have it work normally on the front end
- The usr_roles table should not exist anymore on back end
- The usr_access_roles table should exist on backend

This should be tested with:
- install a new site
- install a site older than 2018-08-02 then upgrade
- install a site newer than 2018-08-02 then upgrade

[Mahara Bot (dev-mahara)]

Patch for "master" branch: https://reviews.mahara.org/9993

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/9993
Committed: https://git.mahara.org/mahara/mahara/commit/a1bce068571b348984d9c2b1b5f8306412f18bd3
Submitter: Robert Lyon (<email address hidden>)
Branch: master

commit a1bce068571b348984d9c2b1b5f8306412f18bd3
Author: Robert Lyon <email address hidden>
Date: Mon May 13 14:09:23 2019 +1200

Bug 1828753: Moving usr_roles table to usr_access_roles table

As this better refects what the table is for

behatnotneeded

Change-Id: Ia6f8746804b50c2e9946d09e0315f5c7cee80ab2
Signed-off-by: Robert Lyon <email address hidden>