Code changes in comment editor

[torgeirl]

As user suggestion for the improved comment editor (#1040): enabling code change snippets in the comment editor to allow examiners to write code changes in their feedback to the student.

Ie..:

#!/usr/bin/env python3

-from jon import dumps as json_dumps
+from json import dumps as json_dumps

where the - line is highlighted red and the + line is highlighted green, similar to how Github allow users to comment inline on pull requests.

[torgeirl]

This could hopefully be added as custom markdown tags when the ACE editor gets replaced (#1217).

[espenak]

Should be usable now I think @torgeirl... This markdown code:

##<##
def Hello(self, x):
    print(f'Hello')
    print('World')
##>##
def hello(self, x):
    print(f'Hello {x}')
    print('World')
##<##

Results in:

Docs

Here is the docs (just screenshot from the builtin docs for the supported markdown):

Why multiple diff "styles"?

This is because the different styles are better/worst for different amounts of changes/code lines. "Side by side" and "simple" is very nice for short examples, while compact is much better for larger snippets with lots of changes. The compact style is very similar (or perhaps the same) as the one the git diff command uses by default. Adding all three was simple since they are supported by the builtin python difflib module.

[espenak]

Here is an a bit more realistic code example with a bit of common code left between the two snippets:

### Simple diff
##<##
def Hello(self, x):
    print(f'Hello')
    print('World')

class HelloWorld(object):
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return self.x
##>##
def hello(self, x):
    print(f'Hello {x}')
    print('World')


class HelloWorld:
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return f'Hello {self.x} world'
##<##


### Compact diff
##<## compact
def Hello(self, x):
    print(f'Hello')
    print('World')

class HelloWorld(object):
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return self.x
##>##
def hello(self, x):
    print(f'Hello {x}')
    print('World')


class HelloWorld:
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return f'Hello {self.x} world'
##<##




### Side by side diff
##<## sidebyside
def Hello(self, x):
    print(f'Hello')
    print('World')

class HelloWorld(object):
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return self.x
##>##
def hello(self, x):
    print(f'Hello {x}')
    print('World')


class HelloWorld:
    def __init__(self, x):
        self.x = x

    def __str__(self):
        return f'Hello {self.x} world'
##<##