UI/UX: Update text throughout the plugin

Text update:
#### Main screen:
- [x] Plugin title change from "Allen Cell & Structure Segmentation DEEP LEARNING" two rows to "**Allen Cell & Structure Segmenter - MACHINE LEARNING**" (bolded is optional) in one single row
- [x] Increase padding between the plugin title and the next text "Segmentation model" from 18px to 30px
#### Curation - Start screen:
- [x] change "Seg 2" to "Seg 2 (OPTIONAL)"
#### Curation - Main screen:
- [x] change the button text "Save Curation CSV" to "Save Curation Progress"
- [x] make sure that guide text "Without merging mask..." under "Merging mask" has the font size of 10, color is hex code #868E93
- [x] remove the guide text "If performing mergin..." under "Excluding mask" as it's no longer the case
#### Training:
- [x] change "Curated image data CSV" to "Curated image data source"
- [x] remove the guide text "See instruction for CSV file" under the "Curated image data CSV" as in this version we don't expect user to generate their curated dataset outside of our plugin, thus we are not providing guidance on this - this might come back in the future
- [x] Add guide text "All values must be multiples of 4" under "Patch size"
#### Prediction:
- [x] change "Image(s) from a directory" to "Image directory"
- [x] remove guide text "Accepts directories..." under the radio option "Image(s) from a directory"
- [x] update "Image input channel" to "Input image's channel"

***
### Size
XS - together might be an S

### Additional notes
V1 deliverable