Improve custom fields feature

[anorthall]

The custom fields feature needs a number of quality of life improvements to be complete:

• Users should be able to view a list of trips with values for certain custom fields
• Users should be able to completely clear a custom field value and delete it, with appropriate warnings
• Users should be able to rename an in-use custom field, with appropriate warnings
• Users should be able to define a custom field as either a distance field, an integer field, or a plain text field
• Statistics app should automatically compile statistics on custom fields based on the above field type.