Support label_name in addition to labels param

What does this MR do and why?

Related to #467387 (closed)

Add backend support for issues label filter param label_name. The old param labels still works but will prefer label_name if provided.

The labels param will be removed in #470402

A few things to note:

• label name in the URL is case sensitive
• no differentiation in selecting project or group label if the name is identical
• project level searches will also return issues with group labels if the name matches

MR acceptance checklist

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Screenshots or screen recordings

No UI difference, but here is a few screenshots of it working

labels passed

label_name passed

label_name and labels passed

How to set up and validate locally

I tested a few variations of labels:

• group label named GroupLabel
• project label named ProjectLabel
• group and project label both named Duplicate

1. setup elasticsearch for gdk, enable, and index everything

2. create group and project labels in a group/project

3. add group and project labels to issue(s)

4. perform a group issues search (search or go to). the labels filter will populate.

5. select one or more labels and apply

   http://gdk.test:3000/search?search=dismiss&nav_source=navbar&group_id=24&scope=issues&labels%5B%5D=249

6. run the same search but replace in the URL labels with the label_name param. Note: the label name is case sensitive

   https://gdk.test:3000/search?search=dismiss&nav_source=navbar&group_id=24&scope=issues&label_name%5B%5D=Duplicate

7. run the same search with both params (will need one of the label ids)

   http://gdk.test:3000/search?search=dismiss&group_id=24&scope=issues&project_id=3&label_name%5B%5D=Duplicate&labels%5B%5D=250