feat: add billing

[djaiss]

Billing is a critical feature.

It allows us to bill customers appropriately, thus giving us more resources to work on the product. We need to make sure billing is done well on day 1.

Here is how it works on OfficeLife.

1. First of all, there is a new .env variable that is enabled by default, which let us have paid plan on the instance. This flag will make sure customers are billed every month.
2. Every day at midnight (UTC), we calculate the number of maximum active employees in the company's account. This job is called LogDailyMaxNumberOfActiveEmployeesInCompanies. An active employee is an employee who hasn't been locked in the account. This information is stored in a new table, called company_daily_usage_history.
3. As additional proof, we will also record, for each day, the employee names and emails that were active in the instance. This information is stored in a new table called company_usage_history_details. This table might be huge in the future as we get more customers - we might need to auto-delete the record after a given number of months.
4. At the end of each company's billing period in the instance, we will get the highest number of maximum active employees in the given time period in the account, and generate an invoice for the company. This information is stored in a new table, called company_invoices. The job is called CreateMonthlyInvoiceForCompany.
5. Once the invoice is generated, we will manually charge the customer and manually send an invoice.

In an ideal world, charging customers will be automated. For now, we'll charge manually.

Here is a breakdown of all the things we need to do:

• Make sure the new .env variable is documented
• Daily job: calculate the number of maximum active employees in the company
• Daily job: record the names of employees that were in the instance each day
• Monthly job: create invoice

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
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92.3% Coverage
0.0% Duplication

[djaiss]

@asbiin please review

[asbiin]

Approved.
Just some consistency changes, because I'm boring.

[asbiin]

This is the same as the commented line below

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!

0 Bugs
0 Vulnerabilities
0 Security Hotspots
0 Code Smells

94.3% Coverage
0.0% Duplication

[github-actions]

🎉 This PR is included in version 0.4.0 🎉

The release is available on:

• GitHub release
• v0.4.0

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[github-actions]

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