Store kernelspecs under jupyter, not IPython
Add kernel directory relative to sys.prefix
Default to systemwide installation for kernelspecs
Update kernelspec command line interface
Update kernelspec tests
add ability to unregister a preset

fix #7218
Revert change to test skip condition

This was part of a commit cherry picked from another branch, and this
change didn't make sense on this branch.
remove kernel selector from header

- add kernel name to indicator
- float kernel logo where indicator used to be
add text placeholder for kernels with no logo

capital first initial on gray field
remove kernel logo text placeholder
transparent gif placeholder src

avoids invalid empty src
abstract some methods in contents service tests

should allow re-use for ContentsManager subclasses
BUG: Set default mimetype when base64 is requested
BUG: Sanitize to_path in ContentsManager.copy.

Without this, one of the test cases in `test_copy` attempts a copy to a
directory that starts with a leading double-slash.
BUG: Allow for the possibility that to_path is empty string.

This is a legal input for representing the root directory.
TEST: Expect a 404 on delete of non-existent file.
TEST: More fine-grained test for directory contents.

Verify that each entry in a directory listing matches the result of a
content-less direct get.
TEST: Add checks for subdirectory name splitting.

Previously the tests failed to catch a bug where a ContentsManager
didn't split name from path in a nested subdirectory.
move symlink tests to TestFileManager
TEST: Refactor symlink tests.