improve the way we fetch platform.js

[Cecilia Vela Gurovic]

We need platform.js for the external video blocktype.

Currently, we fetch (from cdn.embedly.com/widgets/platform.js) and load the file even if we don't have any external video blocks in the view, we only check if the blocktype is installed on the site. We do this every time we load a page.

We need to check:
- Do we need to fetch the file or could we have it as part of Mahara core?
- Is it the latest version of the file?
- We need to load the file only if the view has a video block type in it

[Mahara Bot (dev-mahara)]

Patch for "master" branch: https://reviews.mahara.org/11046

[Mahara Bot (dev-mahara)]

Reviewed: https://reviews.mahara.org/11046
Committed: https://git.mahara.org/mahara/mahara/commit/55a849e4dd0606a42926d848953ae1adeb80fcdc
Submitter: Robert Lyon (<email address hidden>)
Branch: master

commit 55a849e4dd0606a42926d848953ae1adeb80fcdc
Author: Cecilia Vela Gurovic <email address hidden>
Date: Mon Jun 29 16:45:24 2020 +1200

Bug 1885460: load platform.js only for external video block

TODO
embedly documentation indicates to do an async load of the file
https://docs.embed.ly/docs/platformjs

check if it's worth it to allow to define async loads in the
get_instance_javascript inside the blocktype classes
for example
array(
'file' => (is_https() ? 'https:' : 'http:') . '//cdn.embedly.com/widgets/platform.js',
'async' => true,
)

we would need to also modify the get_all_blocktype_javascript function

behatnotneeded

Change-Id: I8ff3cd0b867edbce24c7b2581e6a23e88b68ff0c