Allow to enter replicates with zero concentration

[diegoliberman]

BONUS

1. Reduce the space to enter the number of replicates, so that we gain space for increasing the field for defining the concentration. See this image:

2. Move the instructions label to the line below, closer to the input field

[sardar-usman]

@leandroradusky The 'INSTRUCTIONS' label is not close to input field. @diegoliberman

[diegoliberman]

@sardar-usman good catch! I decided that those details will be fixed in #1909. If you find more, please add a comment into that issue. Thanks!

[ysbaddaden]

@sardar-usman @diegoliberman I extracted the whitespace fix bonus to #1909, so we can move this issue to verified (the bug is fixed).