Convert minutes to seconds for escalation policies

What does this MR do?

There is a blocking bug in the escalation policies feature, where the UI minutes field is not converted to seconds when passing to the backend.

This change ensures that minutes are converted to seconds when saving escalation policies, and vice-versa when loading.

It achieves this with the following:

• Adds a parser/serializer pair to the escalation policy utils.
• Applies these for the get, create and update queries.
• Renames seconds to minutes in all frontend usages, to match the form field.
• Uses the bootstrap-vue number prop for this field.

Out of scope: Adding a new validation message as suggested in the issue.

Issue: #334732 (closed)

Screenshots (strongly suggested)

none

Does this MR meet the acceptance criteria?

Conformity

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• [n/a] I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [n/a] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [n/a] I have tested this MR in all supported browsers, or it's not needed.
• [n/a] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [n/a] Label as security and @ mention @gitlab-com/gl-security/appsec
• [n/a] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [n/a] Security reports checked/validated by a reviewer from the AppSec team