Suffixing field names in auto completion with colon (':'). (Backport of #7489 for 3.2)

[linuspahl]

Backport of #7489 for 3.2

Description

Motivation and Context

The legacy search suffixed field names in the query input's
auto-completion with a colon, as most users would want to write a value
next, after completing the field name. The new search does not do this
yet.

This change is suffixing all field names suggested for field completion
with a colon.

Fixes #7408.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.