Only load template pack JS & CSS assets when needed

[imath]

Currently these assets are loaded everywhere. These should only be loaded when true === is_buddypress().

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The above patch is introducing the bp_enqueue_community_scripts action hook which is only fired when is_buddypress(). It seems to be enough.. But I believe it needs more testing especially with legacy widgets...

[imath]

To follow up with the ​discussion we had during July 6 dev-chat, I've updated the patch to introduce a new filter bp_enqueue_assets_in_bp_pages_only (instead of a constant) we can use to customize how BuddyPress assets are loaded according to 2 options:

1. everywhere (return false to the filter).
2. only in is_buddypress() areas (return true to the filter).

For 11.0.0 the default value will be false: we'll carry on loading BP Assets everywhere. This way we take no risks with BP Plugins which might need these assets everywhere while we let people wishing to contribute to BuddyPress use add_filter( 'bp_enqueue_assets_in_bp_pages_only', '__return_true' ); to restrict BP assets to BP areas only. We'll be able to see if these contributors warn us about broken things (I suspect mainly in BP Legacy widgets).

Before releasing 12.0.0, we'll be able to change our default value to true considering BP Plugins authors had time to check their plugins.

[imath]

In 13306:

First step towards only loading Template Packs assets in BP pages

Avoid loading these assets everywhere inside the blog is a frequent request made by BuddyPress users. In 11.0.0 we are doing a first step into this direction by making available a new filter to restrict Template Packs assets loading to BuddyPress pages.

We encourage these users to simply add the add_filter( 'bp_enqueue_assets_in_bp_pages_only', '__return_true' ); code into a BP Custom file for instance to contribute to testing whether it has an unwanted side effect on the BuddyPress plugins that they might have activated.

This will help us feel more secure about completely restricting these assets to BP pages in a second step.

Props dcavins

Fixes #8679

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It's time to move the other way around!

Let's change our default value to true considering BP Plugins authors had time to check their plugins.

Trac ticket: ​https://buddypress.trac.wordpress.org/ticket/8679

[imath]

In 13418:

Second step towards only loading Template Packs assets in BP pages

In 11.0.0 we made a first step into this direction by making available a
filter to restrict Template Packs assets loading to BuddyPress pages. By
default this filter was returning false to carry on loading assets.

In 12.0.0 we are changing this default value to true so that we are now
only loading Template Packs assets in BP Pages. If you need to keep these
assets being loaded everywhere on your site, you'll have to hook to this
filter & return false.

Eg: `add_filter( 'bp_enqueue_assets_in_bp_pages_only', 'return_false'
);`

Props dcavins

Closes ​https://github.com/buddypress/buddypress/pull/64
Fixes #8679