Added certificate download and verification

[dhruvilmehta]

PR Fixes:

Features added:

1. Download PDF/PNG
2. Certificate verification page
3. Share on Twitter (LinkedIn URl not working)

Resolves #483

Checklist before requesting a review

• I have performed a self-review of my code
• I assure there is no similar/duplicate pull request regarding same issue

Current Certificate design (we can change the template, everything is middle aligned.):

[dhruvilmehta]

This is what the certificate verification page looks like.

Certificates page:

I am not storing any certificates since fetching certificates will not be a frequent task. Instead, I am generating a certificate image every time it is requested.
If you wish to store all certificates then we need to modify some code to store the certificates on the server.

We can also download PNG files to share on LinkedIn/Twitter.
Otherwise, certificates are available on /certificates/verify/{certificateId} , which we are sharing on Twitter/LinkedIn since we cannot directly share png files.

[hkirat]

This needs to change unfortunately since not all purchases are in this db
Some are fetched from appx
Let me take care of this

[hkirat]

/bounty $100