Prevent creation of large _stylesheet class variable

[nenb]

This PR fixes a bug (#337) whereby large stylesheets objects were passed to panel/bokeh to parse and render. The issue was that every time a new message was created in a chat, the same bunch of CSS rules were added to a class variable (_stylesheets), causing rapid growth in the size of the variable.

[pmeier]

To asses the situation correctly:

• ChatMessage._stylesheets is a class variable coming from panel so there is little we can do about it
• It is filled with some default values and we just want to append ours
• Our style sheets are static and thus could also be appended statically

Is that correct? If so, can't we just do

class RagnaChatMessage(pn.chat.ChatMessage):
    _stylesheets = [
        *pn.chat.ChatMessage._stylesheets,
        *message_stylesheets,
    ]

? No need to ever touch them at runtime.

[nenb]

Is that correct? If so, can't we just do

class RagnaChatMessage(pn.chat.ChatMessage):
    _stylesheets = [
        *pn.chat.ChatMessage._stylesheets,
        *message_stylesheets,
    ]

? No need to ever touch them at runtime.

Makes sense to me (although I haven't tested it). Feel free to push this to my branch and use it instead.

[pmeier]

Added and tested my suggestion from #341 (review).