feat(session): delay session boot until all plugins are loaded

[hypeJunction]

In certain situations, we want plugins to have control over the session user,
e.g. by registered custom entity classes. This breaks down the plugin boot sequence
further to allow the system to perform operations, such as session boot,
after plugin files have been loaded, and the necessary plugin registrations took
place

[hypeJunction]

@jeabakker @jdalsem Can you think of any place that we check logged in user before the plugins boot are complete?

[hypeJunction]

Found the answer, there is nothing happening betweeen _elgg_services()->boot->boot() and plugins boot, so we are golden.

[hypeJunction]

Fixes #8868

[jdalsem]

This look good, however having elgg_echo in your static config causes problems (mod/file/elgg-plugin.php). Languages are not loaded in preload. Same could apply to using custom language key in your classes.

Moving translation into preload should be enough i think.

[hypeJunction]

I think allowing function calls in static config should be prohibited.

[jdalsem]

I think allowing function calls in static config should be prohibited.

not something we can enforce now anymore

[jdalsem]

This was done right?

[hypeJunction]

Yes