Allow to set limit of tags returned by TagListResource

[psrok1]

Your checklist for this pull request

• I've read the contributing guideline.
• I've tested my changes by building and running the project, and testing changed functionality (if applicable)
• I've added automated tests for my change (if applicable, optional)
• I've updated documentation to reflect my change (if applicable)

What is the current behaviour?

TagListResource returns all tags matching provided prefix without any limitation. It's especially expensive when we provide a very common prefix e.g. "feed:" or "ripped:"

What is the new behaviour?

• Added count argument to the endpoint
• count is set by default to 20 in "Add tag" autocompletion list