[ENH] Make page selector tracker field respect restrictions on special characters

• Wiki page selector tracker field doesn't accept any more invalid page name
• https://avan.tech/item58090

When some invalid page names are added, and then user tries to create they get an error. Now the trackerlib won't allow tracker item creation, if a page selector field has invalid input value. The global regex pattern for wiki page is used to test page validity. If the user set up a custom validation for the field, it is then be used instead.

An error is being displayed and a tracker item is not created.