Added color of certificate element

[pavel-pasechnik]

Done in this PR:

1. Added a color attribute to the CertificateTemplateEntry model:

   • The color field was added to the database table using migration:
     • Type: string.
     • Default value: "#000000".
     • The field cannot be NULL.

2. Added validation for the color attribute:

   • Checks that color is a valid string HEX code of color (e.g., #FFFFFF or #FFF).
   • The HEX_COLOR_REGEX regular expression is used to validate the format.

3. Implemented the default value setting in the model:

   • If color is not set when the object is created, it is automatically set to "#000000".

4. Updated the CertificateTemplateEntry model:

   • Added validation for the presence and format of color.
   • Used before_validation to set the default value.

5. Updated the database:

   • Performed migration to add the color column with the above parameters.

6. Implemented localization:

   • Added translations for the color field in Russian and Ukrainian:
     • Russian: Цвет.
     • Ukrainian: Колір.

[pavel-pasechnik]

Changelog

• Added support for the color field in SimpleForm via a custom input.
• Created color_input.rb file in app/inputs, which defines the rendering of <input type="color">.