Update GLFM to support iteration autocompletion

What does this MR do and why?

Related to #324298 (closed).

Updates GLFM to support autocompletion for the iteration reference.

In addition, the quick action for iteration should be autocompleted by hitting Tab.

Screenshots or screen recordings

Demo

How to set up and validate locally

1. Prepare iterations. Visit any group and navigate to Issues -> Iterations in the left sidebar.

• Create a new iterations cadence by clicking New iteration cadence.
  • For simplicity, uncheck Enable automatic scheduling
  • To the created iterations cadence, add an iteration.
    • Create a past iteration (due date is in the past) => closed iterations should NOT show up in the autocompletion list.
    • Create an open iteration (start date is in the future)

2. Find a text form that supports GLFM (namely, issue comment)

3. Enter *iteration: to test the autocompletion.

4. Test the autocompletion when using the quick action command /iteration. Type /iter then hit Tab. GLFM should autocomplete to /iteration *iteration:.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.