Update RP post required fields and label requirements

Problem to solve

Some release posts were missing required fields. These missing fields were not being caught by the MR pipeline and had to be resolve after content assembly.

Additionally, in the 18.4 cycle a new Dedicated badge along with gitlab_dedicated field was added. This field was updated manually by the RPM in this cycle but it should move to the individual PMs to determine if their feature is available in Dedicated and correctly fill in the field on future RPs.

Proposal

1. Confirm and add if necessary the following fields as required:

• gitlab_com
• self_managed
• gitlab_dedicated
• available_in with additional requirement that this cannot be contain an empty array.
• documentation_link
• issue_url or epic_url

2. Require that the RP MR has one of the following labels in accordance with the type of MR it is:

• release post itemprimary

• release post itemtop

• release post itemsecondary

• release post itemupgrades

• release post itemextras

• release post itemremoval

  This makes it easier for the RPM to filter and review different categories of RPs.

3. An attempt should also be made to review and perform any updates to handbook page section for available_in guidance.

Implementation

1. Updated required fields in linter: !141182 (merged)
2. Add policy to https://gitlab.com/gitlab-org/quality/triage-ops to ping assignee when missing release post item:: type label and release post item is present
3. Update handbook as necessary