Added Bookmark feature

[ManishShah120]

Description[about the feature]:

• Added the Bookmarking/Saving feature.
• Added an extra dropdown menu under the profile section.

Fixes: #171

Additional Info:

• Converted the post-detail view from a class-based view to a function-based view, so as a result more features and functions can be added to the post-detailed view/page in the future.
• Added an extra separate page for displaying all the saved/bookmarked blogs for any specific user.
• Also added feature to remove the previously saved/bookmarked blog-posts.
• Added a condition for allowing only logged in user to be able to save or bookmarks.

Screenshots/Gif:

[ManishShah120]

@harsh-9in & @Kajol-Kumari , Please have a look.

[ManishShah120]

@Kajol-Kumari , I have resolved the requested changes. Can it be merged now?

[ManishShah120]

@harsh-9in, removed the conflict.