Shortcut for text label overlaps with global command "a"

**Description**
When project has enough labels, one of them can get assigned "a" as a shortcut. However, when using it, it leads to leaving the annotation screen back to the task overview screen.

**To Reproduce**
Steps to reproduce the behavior:
Configure at least 15 labels in text labeling project or assign by hand a shortcut "a" to a label:
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**Expected behavior**
System should not allow assignment of a shortcuts that overlap with global hotkeys:
- it should avoid them in random assignments
- it should complain in a label configuration validation step

**Environment (please complete the following information):**
- OS: Linux/Ubuntu 20.04
- Label Studio Version 1.4.0