Service accounts: Creation logic simplification

[IevaVasiljeva]

What is this feature?

Simplify the creation logic:

• remove one DB transaction to avoid nested transactions;
• add service account to org through CreateServiceAccount method in the user store;
• remove a bunch of code that is not relevant for service accounts;
• move errors to service account model.go;
• use errutil.Error for errors.

Why do we need this feature?

Nested transactions caused issues with instances that have a limit on the number of DB sessions.

Other changes are mainly to simplify the code and make it easier to maintain.

Special notes for your reviewer:

Enterprise PR (a one line change): https://github.com/grafana/grafana-enterprise/pull/4712

[sakjur]

remove one DB transaction to avoid nested transactions

FWIW, you shouldn't have to do that, if there's a transaction on the context object, Grafana should reuse that.

[eleijonmarck]

LGTM

[grafanabot]

The backport to v9.4.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new branch
git switch --create backport-63884-to-v9.4.x origin/v9.4.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x 3fb1894739f0d9344065e4d8b61f5fb896f4c933
# Push it to GitHub
git push --set-upstream origin backport-63884-to-v9.4.x
git switch main
# Remove the local backport branch
git branch -D backport-63884-to-v9.4.x

Then, create a pull request where the base branch is v9.4.x and the compare/head branch is backport-63884-to-v9.4.x.