Default to using first 5 tags on new post instead of 1

[aaroncox]

https://chainbb.com/chainbb-support/@andersrh/using-multiple-tags

There's no reason why it should use the first tag instead of all specified. The end user can choose to remove some if they want (for whatever reason).

[aaroncox]

This will have to be a configurable option for forum owners as well. Some forums use tags differently that causes this use case to be inconvenient.