Debug download.html example.

[ruffin--]

This PR makes download.html [nearly] functional. The previous version did not work.

I've made a few fixes and changes, including adding several convenience features.

1. Fixed typos in downloadfile function
   • The fileid was ignored; value had been hard-coded, which wouldn't work.
   • console.logs referenced "uploads"; now says download.
2. Auth token is now saved in localStorage so that you don't have to get a token each time.
3. access_token was assigned but unused; it's now trivially used.
4. locationid was assigned but unused. It's saved to localStorage now, but is essentially unused.
   • I can't find its use in createClient, eg.
5. listFiles is now recursive through folders.
6. lifeFiles reads a maxdepth from the ui to limit the folder depth used.
   • Default is 2 to include Demo Audio 2.mp3 if using a client_id with all folder access.
7. Message displayed explaining "private" apps if no files are found for the pCloudSampleApp client_id.
8. Double-quotes as requested by .prettierrc.

I realize this is slightly more complicated than a simplest case, but is the minimum I think you could give to demonstrate something approaching real-world usage. At just under 150 lines with html, it's not horrible. It is both folder & file listing plus download.

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client_ids and private vs. all folder access

I noticed that there were two client_id values being used throughout the example folder:

1. new_APP, client_id of "3dunsTvYJsu", ALL FOLDERS access
2. pCloudSampleApp, client_id of "p1WznE2dEPm", private access only

I included both in the code here, though the pCloundSampleApp client_id is hooked up by default. That does mean a user needs to put files in its app folder, which is why I added the message in case the app-specific folder is empty, as mentioned above.

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Strange server behavior

There are a few other things I'd clean up in a perfect world. I'd remove parent-scoped variables and change variable names to camelCase, for instance. But this seems to work fairly well, except...

The download doesn't seem to work! There's a CORS error, which means pCloud would need to change settings on the API side for this to work from a browser using the example code, even fixed.

It'll work from node, of course, or something else that isn't worried about CORS, but not from a browser on a non-pCloud domain.

download.html:1 Access to XMLHttpRequest at 
'https://vc828.pcloud.com/[value removed]/Demo%20Audio%202.mp3' 
from origin 'http://127.0.0.1:8080' has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header is present on the requested resource.

I'll open an issue for that and reference it here. EDIT: Looks like it's been open a while, and the answer is that getfilelink isn't supposed to work with CORS. :sigh:

If true, we should probably remove download.html, right?

I've also noticed I have to sign-in twice in the OAuth interface to get a token with the examples. That's strange, though it doesn't prevent the app from eventually functioning.

Thank you!