feat: Attaching tickets+invoice to order emails

[mrsaicharan1]

Fixes #5888

Checklist

• I have read the Contribution & Best practices Guide and my PR follows them.
• My branch is up-to-date with the Upstream development branch.
• The unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

• All the functions created/modified in this PR contain relevant docstrings.

Short description of what this resolves:

Order Invoices and Tickets are attached to the emails being sent out when the orders are placed. Currently, we are manually sending an API request to SendGrid which makes it hard to add any new headers, attachments or any new data. The SendGrid Client makes it easier to do this.

Changes proposed in this pull request:

• Order Invoices and Tickets are attached as PDFs
• Made the email message non ambiguous.
• Switched to the SendGrid Python Client to support multiple attachments.

[mariobehling]

The problem with this approach seems to be that we make ourselves entirely dependent on Sendgrid, which we don't want. We actually want to move away from Sendgrid. We want to be able to send out information no matter what gateway we use. Sendgrid is just one. A better option for now (as we are small) is to use SMTP.

[codecov]

Codecov Report

Merging #5917 into development will decrease coverage by 0.08%.
The diff coverage is 18.42%.

@@               Coverage Diff               @@
##           development    #5917      +/-   ##
===============================================
- Coverage        66.48%   66.39%   -0.09%     
===============================================
  Files              286      286              
  Lines            13933    13958      +25     
===============================================
+ Hits              9263     9268       +5     
- Misses            4670     4690      +20

Impacted Files	Coverage Δ
app/api/helpers/system_mails.py	100% <ø> (ø)	⬆️
app/api/helpers/mail.py	28.73% <100%> (ø)	⬆️
app/api/helpers/tasks.py	17.59% <19.04%> (+0.44%)	⬆️
app/api/orders.py	34.21% <6.66%> (-1.28%)	⬇️

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[mrsaicharan1]

I’ve chosen this approach because this is being used in production right now. I will implement this for SMTP too. I just did this so that we could have a working system for now.
Also, as soon as this is good to go, will integrate a feature into the system which falls back to SMTP if sendgrid API calls exceed our quota

[abhinavk96]

Faulty commit history, can't merge

[mrsaicharan1]

Faulty commit history, can't merge

Yeah , was fixing it. Will update.

[mrsaicharan1]

@CosmicCoder96 @iamareebjamal Fixed conflicts. Please check.

[abhinavk96]

@mrsaicharan1 a codacy issue is left.

[mrsaicharan1]

@CosmicCoder96 @iamareebjamal Fixed Codacy & Hound Issues.