Issue 1422  Delete Language Pack Button

Sorry to Generate a new PR , by mistake closed the previous one , 
You can find it at  https://github.com/learningequality/ka-lite/pull/1711/

<b>In  api_views.py </b>

I have used try expect because we have to delete multiple folder and we are not sure that folder will exist or not. So , if exception comes on first folder other won't be checked , to handle that issue ,I am storing that exception in a list & continues the loop without raising it  and in the end if exceptions are there that list is printed to user as Json error message containing all errors.

<b>In JS file : </b>
we need to call this functions 
get_installed_languages();
display_languages(installables);

whether we get success or fail . Because we can encounter error if  srt or exercise directory of language pack doesn't exist . In this case locale directory of that language pack will be deleted , so that language should be removed from installed language list , even we get an error. So, need to refresh the drop down and installed language list on the page.



<b>In browser_tests.py </b>

As Aron suggested 

In  add language pack
first, check if "de" is not in the list
and then download it
and then see if it's in the list now

Opposite for Delete 

But i have doubt about them : 

We are using reverse urls so , 
  start_languagepack_download is added in api_urls.py , do we need to add it somewhere also ?
  delete_language_pack url is added in api_urls.py , do we need to add it somewhere also ?