Describe the bug

1. When clicking over the "Sign" button in Initiatives the button changes color but it doesn't change text, the button changes from red to green, yet it says "sign". Participants not familiar with color-codes, when already signed, will see a green button saying "sign" and they will click it to sign, to see that it changes to red, without knowing what that means.

2. Together with a change of color the button text should change to "Already signed" when participants click on it.

3. I found this bug in meta.decidim.org initiatives. v. 0.15

https://meta.decidim.org/processes/bug-report/f/210/proposals/13350

Screenshots
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Extra data (please complete the following information):

• Device: [e.g. iPhone6, Desktop]
• Device OS: [e.g. iOS8.1, Windows 10]
• Browser: [e.g. Chrome, Firefox, Safari]
• Decidim Version: [e.g. 0.10] 0.15
• Decidim installation: [e.g. MetaDecidim]

Additional context
Add any other context about the problem here.