As admin, I can restrict execution of tests only to the test run assignee

It should be possible to restrict the execution of the tests only for the Test Run assignee. This should be a global Xray configuration and should be enabled by default for new installations. Current Xray installations must remain with this setting disabled.

ACs:

• Project admin can execute tests either the setting is enabled or disabled.
• When this setting is enabled, only assignee users can execute Test Runs.
• Other users should be able to access the execution screen in readonly mode.
• Only the Test Run assignee can start executing a Test Run.
• If a TestRun has not started yet, other users must not be able to go to the Test Run screen, not even in readonly mode.
• If TestRun is unassigned, the TestRun is not executable by a "current" user
• A new global miscellaneous setting is provided on the Xray configuration enabled by default for new installations.
• Current installations of Xray should display this feature as disabled.
• Other users must be warned that they are not assigned to a given Test Run.
• When other users enter the execution page in readonly mode, the readonly message should display the cause for why the TR is readonly.
• Backend must also verify the restriction, thus, if a browser window has a non updated view and the user try to execute a test run without having permission an appropriate message error must be presented.
• REST API call do not restrict to assignee.