Add initial Event Message Template previewability

[eileenmcnaughton]

Overview

Add initial Event Message Template previewability.

Note this PR also makes participant and event tokens work in the event_online template - they were only working in the offline before this

Before

No event template preview possible

After

Initial functionality added - event tokens are an easy add here to I fixed some very simple variables to be tokens to get this far

Technical Details

This adds if for online & offline receipt. The other participant templates will be a follow on once this is merged

Comments

Pre-existing test cover in https://github.com/civicrm/civicrm-core/pull/24323/files#diff-c4ddaa8aa71ca49d31466c04cefbb2f1d608a04e3836791a756859d73ef560c2

@demeritcowboy hopefully this is an easy-ish merge as it is similar to other recent ones

[civibot]

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[demeritcowboy]

Is it expected that there's no fee information in the preview? This is just to get it working?

Otherwise this looks ok just some notices that I think were there before too.

[eileenmcnaughton]

@demeritcowboy yep - that's expected - it's just a very limited initial add

[demeritcowboy]

okey dokey

[eileenmcnaughton]

thanks