199 Update on some CSS of components and layout structures

[lucianosimoni]

199 updates:

CohortsPageTeacher.js and .css

• Removed Header from CohortsPageTeacher.js - Was not necessary anymore.
• Wrapped within a
  with the cohortList className.
• Removed page border and margin-right (Could not see the reason to have it)
• Setted prop renderAllButton from the StudentsList to false

StudentsList.js & list.css

• Gave it a border radius, padding and margin of 16px
• gap to to the container - Between header and list
• Simple hover animation on the students item.
• Changed width of add and all button to be fit-contet (Avoids word over word)
• Added hover animation in the all and add button.
• removed: min-width and position, wasn't necessary.
• reduced padding-bottom to be 8px (was 50px previously)

StudentListItem.js

• Moved img element inside , so the user have a little more space to press the student.

MyExercises.js

• Added a <span> to be rendered if there is no Exercises for that cohort

TeachersList

• Updated css

CohortsList and CohortsListItem css.

• Major updates related to css.

CSS tested with responsive layouts. Working just fine - Not very small screen although.