Disable personal data export and personal data erasure for multisite (for now)

[allendav]

For 4.9.6, disable personal data export and personal data erasure. We're going to properly add them to a subsequent release in #43738

[allendav]

Do not add erasure and export menu items and short circuit erasure and export pages on multisite installs

[allendav]

Make the disabling of export and erasure filterable

[iandunn]

In 43085:

Privacy: Limit export and erasure to super admins on Multisite.

Multisite networks have a variety of use cases, and in many of them single-site administrators are not trusted to take actions that affect the whole network, require making decisions about legal compliance, etc. By default, those actions should require super admin capabilities. Plugins can be used to override that behavior if a particular site's use case calls for it.

Props allendav, jeremyfelt, iandunn.
Fixes #43919.

[iandunn]

Reopening for backport to the 4.9 branch.

[SergeyBiryukov]

In 43111:

Privacy: Limit export and erasure to super admins on Multisite.

Multisite networks have a variety of use cases, and in many of them single-site administrators are not trusted to take actions that affect the whole network, require making decisions about legal compliance, etc. By default, those actions should require super admin capabilities. Plugins can be used to override that behavior if a particular site's use case calls for it.

Props allendav, jeremyfelt, iandunn.
Merges [43085] to the 4.9 branch.
Fixes #43919.

[desrosj]

Moving to the new Privacy component.