Display custom role name in invitation emails and notifications

What does this MR do and why?

When a custom role is assigned, display the custom role's name in access granted notifications.

Issue: #455486 (closed)

Screenshots or screen recordings

Group members

New users

Invitation email

	Before	After
HTML
Text

Invitation accepted email

	Before	After
HTML
Text

Invitation accepted notification

Before	After

Existing users

Access granted email

	Before	After
HTML
Text

Project members

New users

Invitation email

	Before	After
HTML
Text

Invitation accepted email

	Before	After
HTML
Text

Invitation accepted notification

Before	After

Existing users

Access granted email

	Before	After
HTML
Text

How to set up and validate locally

As admin:

1. Create a top-level group, a project within that group and apply the Ultimate license to the group
2. If SaaS mode is:
   • off: Visit http://localhost:3000/admin/application_settings/roles_and_permissions and create a custom role with the Manage deploy tokens permission enabled, based on Guest access
   • on: Visit http://localhost:3000/groups/${new_group}/-/settings/roles_and_permissions and create a custom role with the Manage deploy tokens permission enabled, based on Guest access
3. Invite a user to the group and assign the new custom role to the user
4. Invite a user to the project and assign the new custom role to the user