[issue tracker] batch edit enhancements

[regisoc]

Brief summary of changes

This PR adds batch edit enhancements, already included to HBCD:

• enables markdown in batch edit description and comments.
• adds some CSS adjustments such as checkboxes aligned with label, filters items in grid layout, and wraps possibly long dropdown items when editing an issue.
• adds a 4th filter tab on possible assignees.

Tests

• Go to Menu > Tools > Issue tracker module
• Go to batch edit tab.
• Add/modify a comment/description with markdown e.g. formatting, link. and see the formatted result.
• Check assignees in assignee filter tab. and see the filtered issues.