Use EF Core 2.1 Seed feature in "Create an MVC web app" tutorial

[dannevesdantas]

Closes #9205

Updated the Create an MVC web app tutorial to use EF Core 2.1 Seed feature in MvcMovieContext instead of a separate SeedData class. The goal is to reduce the amount of code necessary to seed data and consequently decrease the complexity to beginners, also making the docs more didactic by introducing the EF core 2.1 seed feature.

• Updated the v2.1 sample project code to seed data in MvcMovieContext through EF core 2.1
• Added a migration named SeedData in the v2.1 sample project to be able to insert the seed data in the database.
• Updated the Rating existing migration in the v2.1 sample project to deal with seed data in MvcMovieContext.
• Updated the Seed the database tutorial’s section to seed data in MvcMovieContext class instead of SeedData class (only in the 2.1 docs version).
• Updated the Add a new field tutorial’s section to add the Rating field to the MvcMovieContext class instead of the SeedData class (only in the 2.1 docs version).
• The asp.net core <= 2.0 version of the tutorial continues using the SeedData class as it is. No changes in the asp.net core <= 2.0 sample project or tutorial instructions.
• Split the Seed the database tutorial’s section into two different files (seed-db.md and seed-db-21.md) to improve docs maintenance and markdown code readability.

Feedbacks are welcome!
Thanks so much!

[dnfclas]

All CLA requirements met.