Add seed data for the very first time installation

[pournasserian]

When a developer clones the project, only connection string (or other few configs) should be set.

Then by running the application, initial seed data should be inserted to the DB:

• Site for localhost
• User
• Role
• A few Pages (About Us, Contact Us, etc.)

The above objects should be read from json files which exist in the project's source files (like SeedData/user.json).
It is not preferred to hard-code the initial/default objects' properties in the C# code.
So, in the first request we should check if any Site records exists or not.
If not, it should read the site.json and insert to DB, then user.json, role.json, etc.

Here is a draft hard-coded seed data implementation:
https://github.com/ubeac/ubeac-cms/blob/main/uBeacCMS.Web/SeedData.cs

And the usage in program.cs:
app.Services.SeedDefaultData();