Activity: Add Embeds support to single activity items.

Much like the Post Embeds feature introduced in WordPress 4.4, this commit
adds support for single activity URLs to be auto-discovered by oEmbed
consumers.

This allows activity URLs to be rendered into an embeddable format on
third-party websites.

Specifically, this commit:

• Introduces the BP_Activity_oEmbed_Component class, which hooks into WordPress' oEmbed provider API via the BP_oEmbed_Component class.
• Renders oEmbed requests for single activity items into a template suitable for embedding.
• Introduces embed template parts into bp-legacy - /bp-templates/bp-legacy/buddypress/assets/embeds/

See #6772.