add text field with placeholders for custom label format

[TomW1605]

implement #78 and #231

this still needs some work.

• there are some translation strings that need to be added (i will do english, not sure about the others)
• need to add some filtering to only allow a set of html tags
• need to localise the number format functions (eg, use the right money symbol and ./,)
• need to add a date format function
• need to add an expandable list of placeholders with examples (also want to make it so if you click one it will be inserted into the text)
• need to add explanation/examples of how the conditional prefix/suffixes work
• work out how to handle extra fields