Adjust find_by_instanceid_username() function to allow check remote then fallback to usr table

[Robert Lyon]

Currently the find_by_instanceid_username() function either only checks the remote user table (auth_remote_user) or it checks the user table (usr)

It would be more useful if it could do the following:

When $remoteuser flag is set:

- Check remote user table and return user if a match found
- But if no match found then fall back to check user table

When $remoteuser flag is not set:

- Just check user table

[Mahara Bot (dev-mahara)]

Patch for "master" branch: https://reviews.mahara.org/10243

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/10243
Committed: https://git.mahara.org/mahara/mahara/commit/977986426db4093955a9a60307a37145a4227325
Submitter: Robert Lyon (<email address hidden>)
Branch: master

commit 977986426db4093955a9a60307a37145a4227325
Author: Robert Lyon <email address hidden>
Date: Tue Aug 6 09:14:47 2019 +1200

Bug 1839032: Adjust $user->find_by_instanceid_username()

Currently it checks either
- remote user table
or
- the user table

Allow it to check either
- remote user table
-- then user table if not found in remote user table
or
- the user table

Change-Id: Iaa5a59ea056a2a4ce40df324b0f084d0fcd941d8
Signed-off-by: Robert Lyon <email address hidden>