Educator app refactoring from students to multiple account types [8,13,0.62]

[stenington]

#2430 described a tightly scope refactoring to do within the educator app, to fix a problem where a template talked about students but actually handled both student and family accounts. (This was done in preparation for making the same template handle educator accounts as well.)

There are more places where the educator app talks about students, despite handling multiple account types. TBD: list out where the language used in code, urls, etc. needs to be adjusted to be accurate.