More powerful Inline Admin `extra` parameter (#19425)

Right now, the number of extra inline forms in inline admins is
not that flexible. It should be  possible to define a method
which can decide the number based on the context.

Signed-off-by: Rohan Jain <crodjer@gmail.com>
Add missed test for inline form #19425

Add the test case for verifying `get_extra` implementation
for binary tree admin.
Missed staging the file on previous commit.

Signed-off-by: Rohan Jain <crodjer@gmail.com>
Inline admin `extra` option for back to validations #19425

Signed-off-by: Rohan Jain <crodjer@gmail.com>
Use string formatting instead of `str` for compatibility

Responding to comment: https://github.com/django/django/pull/576#discussion_r2521125

Signed-off-by: Rohan Jain <crodjer@gmail.com>
Add documentation for `InlineModelAdmin.get_extra`

Ref: #19425
Signed-off-by: Rohan Jain <crodjer@gmail.com>