Add priorities to quickstarts within bundles

[randomnetcat]

Companion PR to RedHatInsights/learning-resources#65.

This makes a few changes:

• Add a priority field to the relationship between a quickstart and a tag, which can only be set on a bundle tag.
• When quickstarts for a single bundle is requested, sort the returned quickstarts in ascending order of priority (defaulting to 1000). Note that this doesn't handle the case of multiple tags because the ordering that should be used would be unclear (and because the learning-resources UI only cares about single bundles (https://github.com/RedHatInsights/learning-resources/blob/cf6fc78ad7ddeed19576e4b0eaee318c9b0fbb92/src/hooks/useQuickStarts.ts#L94-L96).
• When seeding quickstarts, add all bundle priorities in a new bundle_priority metadata tag, so that the learning-resources UI has a chance to examine them (see the linked PR).

Priorities are set by adding them to metadata.yml, e.g.:

kind: QuickStarts # kind must always be "QuickStarts"
name: rbac-configuring-access
tags: # If you want to use more granular filtering add tags to the quickstart
  - kind: bundle
    value: settings
  - kind: bundle
    value: iam
    priority: 1100