Add Collections Column to Tag Page

### Current Behavior

Dependency-Track v4.13 (currently available as SNAPSHOT) introduces "Collection Projects" that are "basically projects used as parent for other projects that shall not hold any own component or vulnerability data, but instead get calculated from child projects using different configurable aggregation logics." See PR #658 for more information, including screenshots.

Aggregation logic includes "direct children with custom tag"

There is thus a need to track the big picture of where tags are used in collections.

Use cases:

* I want to replace two tags by a single tag (ie, merge) and therefore need to get an accurate idea of usage of these tags before proceeding
* My standard for collections configuration is to use "Latest Version"... I thus want to see whether there are exceptions to this. eg, using tags instead

### Proposed Behavior

Add a "Collections" column to the tag screen (`/tags`)

The column would display a count for how often a tag is used in project collection logic...


### Checklist

- [x] I have read and understand the [contributing guidelines](https://github.com/DependencyTrack/dependency-track/blob/master/CONTRIBUTING.md#filing-issues)
- [x] I have checked the [existing issues](https://github.com/DependencyTrack/frontend/issues) for whether this enhancement was already requested

Start at the /tags screen and read PR #658 for the collection-project behavior and aggregation context. Trace how tag usage is displayed there, then verify that the new Collections column shows the count of collection-logic uses for each tag, including tags used through the stated aggregation logic.

Written by the indexing model from the issue text.