Introduce related alerts to incidents
### Proposal
1. Change some of the verbiage around Linked Issues for the Incident Issue type (all changes are also captured in the designs below as well!):
- Existing `Linked issues` text becomes `Linked incidents, issues & alerts`
- When a user clicks on the `+` sign and expands the form, `The current issue` text changes to `The current incident`.
- In the same expanded form, `the following issue(s)` becomes `the following incident(s) or issue(s)`.
- An additional field is added for inputing alerts. This field is displayed by default, and can be hidden if `blocks` or `is blocked by` is selected in the form radio buttons.
2. Allow for Alerts to be linked in addition to related Incidents and Issues to the Incident Issue type.
### Designs
| Default state | Adding related content |
| ------ | ------ |
| ||
_**Note: the related issue and related merge request widgets are currently being unboxed in !87886. We should plan on matching the styling of this new links widget to whatever work is done in that MR. [Here's](https://www.figma.com/file/cNfWMYHqi3Qnq4KMDcIQM5/Related-incidents%2Falerts-widget-gitlab-com%26356057?node-id=749%3A52352) how that might look, depending on how the design is finalized in that MR.**_
[Figma](https://www.figma.com/file/cNfWMYHqi3Qnq4KMDcIQM5/Related-incidents%2Falerts-widget-gitlab-com%26356057?node-id=749%3A54154)
### Intended users
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### Metrics
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Increase Monitor - SMAU, Unique users that interact with Alerts and Incidents AND Monitor - AMAU - Alerts Active Users
### Implementation
- [x] Stop syncing alert & incident statuses entirely (or showing an alert's escalation policy on the incident page).
- [x] Allow incident escalations on any incident, regardless of alerts.
- [x] Swap issues to allow many associated alerts. (`has_one` -> `has_many`)
issue