Prevent and fix tags starting with a hash-sign

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On answers.ros.org there are a lot of tags starting with '#', resulting in redundant tags like navigation/#navigation, trashing the tag subscriptions etc.

Additional info mentioned in the local issue on github:

If one person makes this mistake the tag gets added to the database. Once it's in the database it goes into the suggested tags drop down. And because the # alphabetized earlier people see the #xxx version first and select it instead of xxx.

I've manually fixed all the tags on answers but they remain in the suggestion drop down.

Two improvements possible:

• Sort the tag suggestions by their popularity instead of alphabetical.
• Do not display tags with zero usage count.