Enhancement: delete unused topics in studies

I would like to see a feature enhancement, so the [Contributing Guidelines](github.com/ornicar/lila/blob/415ce7e1d3d66ab4678ec3a426251214057e595e/CONTRIBUTING.md) say to start a forums post about that and to discuss it with others. What do you think about that change?

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We have collected a lot of studies, so we started to use topics to manage them. Topics are created each time I enter a string in the topic dialog. From time to time, there is a spelling error, or the wrong term used, or just a bad term you delete then later from the study. But a one time used term will stay there forever, even if no study any more uses that term.

What I would like to see here is:

* A term that is not used in any study any more is automatically deleted.
* or / and Allow to delete a term manually, e.g. by providing a small delete button besides the studies overview at each topic.
* or / and Make that delete button dependent on the state of the topic: is that topic used in any study, don't allow to delete it.
* or / and If a topic is used in some studies, and the user wants to delete it, automatically remove that topic from all studies that reference it.

I think this implementation should be not too hard, and avoids collecting unused topics over time.

Hi,
I like your idea, however there are some things where I don't agree:

* A term that is not used in any study any more is automatically deleted.

I like this idea, it would make incorrect topics eliminated once they are deleted.

* or / and Allow to delete a term manually, e.g. by providing a small delete button besides the studies overview at each topic.
* or / and Make that delete button dependent on the state of the topic: is that topic used in any study, don't allow to delete it.
* or / and If a topic is used in some studies, and the user wants to delete it, automatically remove that topic from all studies that reference it.

I think this ideas are worse than the first, because it would be confusing to the user to distinguish between removing a topic from all places and removing it from their own study, also the third gives too much control to the user that uses a topic for the first time, if a user was the first to use the topic "Endgame" the user shouldn't be able to remove the topic.

I think other ideas would be:
* add a small number indicator to know how many times has a topic been used
* create some standard topic groups for most popular topics and make them redirect to one, in example:
create a topic group: "Openings" that includes "Opnings" "Opening" or other variations, and when the user uses one of the variations should be redirected to the original topic

I just noticed that a similar feature is already there. When you go to your studies, you see on the left among a lot of filters the filter named "Topics". When clicking on it, you have at the top: "Topics to organize your studies with" and a big box with all topics in. There you can delete topics and save them.

Problem solved for me, that is sufficient. And there you will find as well Popular topics, so you can use some of them.