Support user defined global functions

[BenSurgisonGDS]

See Feature suggestion: support user-provided functions as well as filters

• added support for user defined functions
• added tests for user defined filters and functions for app and plugins

[joelanman]

#808 (comment)

Personally I would not want to call this file 'globals' as it is only for the view, not routes, and it's specifically for functions, not other data types. So I'd suggest functions.js

[joelanman]

will discuss naming of globals with @nataliecarey

[nataliecarey]

I've caught up with @joelanman about this. We've had on-and-off discussions about how to support globals (variables/constants that are available everywhere, not just nunjucks) and he's raised a concern that this change might limit our options further down the line.

Please can we do the following in this PR:

• Change the interface from addGlobal to addFunction
• Check the type of the input to make sure it's a function

This means that we're providing the same behaviour to our users but leaving room to deal with something like addGlobalValue later without risking interface conflicts.

[nataliecarey]

I don't think we should be putting a globals starter in.

[BenSurgisonGDS]

Removed

[BenSurgisonGDS]

I've caught up with @joelanman about this. We've had on-and-off discussions about how to support globals (variables/constants that are available everywhere, not just nunjucks) and he's raised a concern that this change might limit our options further down the line.

Please can we do the following in this PR:

• Change the interface from addGlobal to addFunction
• Check the type of the input to make sure it's a function

This means that we're providing the same behaviour to our users but leaving room to deal with something like addGlobalValue later without risking interface conflicts.

Done

[BenSurgisonGDS]

will discuss naming of globals with @nataliecarey
changed as requested

Change has been made as requested and approved by someone else