Update MUI inputs to outlined

[j-or]

All text fields and selects should be outlined (the new default in MUI). Some places where this affects the layout:

• Input fields in data points table (especially with many vars)
• Plus-button in categorical vars
• Select dropdowns in data points table

[langdal]

@j-or Is this issue fixed by #183

[j-or]

@langdal Yup