refactor: certificate ui & add completedAt field

[Gautam-Hegde]

Context: Since we are generating certificate of type PDF ( using pdf-lib ) & PNG ( using HTMLCanvas api ) both have few standard fonts exposed for use. Either we can have a cursive font on PNG certificate but not on PDF which is inconsistent.

So in this PR I've used an overlapping web-safe font exposed by both of them i.e. Courier

Using a external font locally is causing loader(webpack config) issues.

• This PR also adds completedAt field of type Date in the schema and lastly used legit signature on the cert(photoshop) ;)

Resolves #596

Checklist before requesting a review

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