Gradebook single view loads all users into memory causing out-of-memory errors

As identified in MDL-48273, the single view report loads every single user with active enrollment in the memory. This leads to two main problems:-

1. High memory usage.
2. The ui to select user becomes an extremely long drop down for large courses.

Suggested changes:-

1. Show a search user interface instead of drop down when Number of users exceed a certain value say X.
2. Load only the required users in memory at best this should never exceed X