Add links "plugins" & "templates" to intro_search.txt

Safari 9 in iOS does not support the use of `const` in strict mode. Unfortunately, some of us are stuck with Safari 9 due to not being able to upgrade.

I am a big fan of progressive enhancement, but because all javascript code is packed together and sent off to the client, this small incompatibility breaks all javascript for safari 9 browsers. Switching these two keywords makes everything work again.

There are no other uses of `const` in the codebase as far as I can find. This change has little to no impact on anything else, but slighlty extends the range of supported browsers.

Plugins may want to add further fields to be present when saving drafts.
Plugins may want to execute some js functionality that should be timed to
the saved draft/refreshed lock.

If a plugin does another init() to attach the dw_locktimer to its own editor,
then the default callback would be added a second time, causing unexpected
and undesired behavior.

This includes the changes from the following commits:
6ef45cc1
6ca947f3
0fff419c
87bed8b6