feat(eventtemplates): enable custom event template UI to display when no target is selected

[andrewazores]

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Related to #225

Description of the change:

1. Allows the Event Templates view to be displayed even when there is no target selected. This allows the user to view, add, or remove Custom Event Templates without needing to select a target - particularly useful when there aren't any targets currently discovered.
2. Similarly, the Agent Probe Templates view is displayed when there is no target selected.
3. Fixes a similar bug on the Automated Rule creation form where the event template selections were limited to the set intersection of those available on the matched targets. If there were no matched targets (including if no targets are discovered), then the event template list would be empty. Now, if there are no matched targets, the non-target event template list (ie the custom templates, and the upcoming preset templates) will be provided as options.
4. Fixes an API V4 bug (oversight) where custom event templates cannot be uploaded

How to manually test:

1. Build and run Cryostat with this PR
2. Go to Events view
3. Do not select a Target
4. Ensure that Event Templates and Agent Probe Templates views are visible, and that templates can be listed/added/removed from each
5. Select a Target, creating one (localhost:0) if necessary
6. Repeat step 4, and ensure that target-specific views (ex. Event Types) are now also interactable as normal