UI Template - Run generic JS onMounted

[joepavitt]

Description

• We group any generic JS we find into a beforeCreate function, which was then run in the beforeCreate lifecycle hook of VueJS. This was causing problems though if a user was trying to reference HTML elements in their component e.g. https://discourse.nodered.org/t/graph-shapes-and-single-point/90041/4 as the component hadn't yet been mounted.
• This PR moves the beforeCreate content (i.e. generic JS) into a new hok called js, and runs this code in mounted() instead so that the code can reference HTML content

[Steve-Mcl]

Approved with an optional comment update.

One (probably naive) question though, why mounted and not beforeMount?

would beforeMount not give users an opportunity to "do stuff" before its displayed? (or would it simply not work due to not being present in the DOM)?

[joepavitt]

(or would it simply not work due to not being present in the DOM)?

This - beforeMount would still not permit code to reference elements in the DOM